1. Field
The present disclosure generally relates to optical-detector circuits. More specifically, the present disclosure relates to an optical-detector circuit that includes an ultra-compact photodetector implemented on top of an optical waveguide.
2. Related Art
Optical interconnects integrated with complementary metal-oxide-semiconductors (CMOS) can be used to provide on-chip and off-chip interconnects with communication bandwidths larger than a terabit-per-second. One of the key components that are needed to implement these optical systems are photodetectors (PDs). Typically, PDs are monolithically integrated in the same silicon layer as the transistors, as well as other optical and electronic components. Germanium is often used in PDs because it is an efficient absorbing material at infrared wavelengths (including the 1.5 μm band), and because it is available in the front-end CMOS manufacturing.
In general, there are two main configurations for PDs: surface-normal PDs, where the input optical light is directed perpendicularly to the PD top surface; and waveguide PDs, in which the optical light is coupled into an optical waveguide that is terminated with light-absorbing media to convert signals from the optical domain to the electrical domain. These two configurations have been demonstrated in silicon/germanium material systems with application in silicon photonics. In high-speed (but less than 10 Gb/sec) systems, the surface-normal PDs are usually either mesas or planar devices having a circular light-sensitive aperture with a 10-40 μm diameter. However, because the optical input fiber typically needs to be accurately aligned with the aperture, it is often difficult to reduce or scale down the physical size of the optical input fiber without complicating the mechanics of the optical-input-fiber light coupling.
Either of these PD configurations can be butt coupled or evanescently coupled to the absorbing germanium section. Evanescently coupled waveguide PDs are typically 20-40 μm long, with widths ranging from 500 nm to several microns. Note that the physical size of these PDs defines their effective absorption length, which usually cannot be further reduced without significantly decreasing the optical responsivity of the PD. Also note that a waveguide PD is often preferred over a surface-normal PD in a tightly integrated photonic on-chip system.
Because PDs with extremely low dark current are typically needed for high-fidelity systems, the germanium growth usually is carefully optimized to obtain high crystalline-material quality (which affects the dark current and the PD sensitivity). Furthermore, in order to minimize the bulk contribution to the dark current, germanium PDs are often mono-crystalline, with low defect density and low impurity levels. In addition, because the germanium sidewalls also contribute to the surface component of the dark current, the dark current is directly linked to device size. Consequently, increasing the area and/or the perimeter of a device usually increases the dark current, so minimizing the device size (without sacrificing the optical responsivity) typically improves performance.
Note that the capacitance of a PD influences its electrical bandwidth. In particular, devices with larger capacitance have slower response times. Because capacitance is proportional to the device area and inversely proportional to the device thickness, once again minimizing the device area typically increases the electrical bandwidth and, thus, has a positive effect on the performance of a PD. However, it is often difficult to modify the device thickness without affecting the optical responsivity. For example, in a surface-normal PD, a device thickness of nearly 3 μm is usually needed to absorb most of the incident optical photons. In contrast, in a waveguide PD, the device thickness can be reduced to 100 nm because the evanescent absorption on this length scale takes place in the optical-waveguide direction with an effective propagation length of 20-40 μm. Therefore, for a similar device area, a waveguide PD often has a larger capacitance and a smaller electrical bandwidth because of a thinner germanium junction.
Moreover, the photonic-component integration in the optical systems is often implemented in a hybrid fashion. This can be economically and/or technologically beneficial, especially when integrated components have to be manufactured separately because of incompatible processing conditions. For example, high-performance PDs and lasers can be efficiently fabricated using III-V materials on indium-phosphide substrates, while optical waveguides and passive-filtering components can be fabricated using silicon-on-insulator (SOI) technology.
The hybrid integration of these components (i.e., III-V to SOI or SOI to SOI) often requires routing optical signals from an optical-waveguide layer into the III-V based components. This routing of the light has been demonstrated using mirrors formed in SOI optical waveguides. In these existing approaches, the optical waveguide is terminated with 90° sidewall, and a tilted interface is micro-machined farther along the optical path to route the optical signal out of the plane. Note that this optical path includes the distance between the terminated optical waveguide and the mirror, and from the mirror into the PD (or a matching mirror). However, for a typical configuration, this optical path is several to tens of microns long, which often results in signal loss due to optical diffraction.
Hence, what is needed is an optical-detector circuit that does not suffer from the above-described problems.